This Application claims benefit of German Appln. Number 19924676.9 filed on May 29, 1999.
Not Applicable
The invention relates to an endoprosthesis for the human hip joint. Artificial hip joint assemblies are used in surgery and orthopaedy when the hip joint proper has been destroyed because of diseases, wear or injuries and gives pain when in function.
As a rule, a resection is then made on the destroyed joint portions and an artificial hip joint assembly is implanted. This hip joint assembly, as a rule, is made of plastic and metallic components. The plastic material, as a rule, is polyethylene whereas the metals are forged steels and, especially, titanium alloys.
Whenever the artificial hip joint assembly is moved in the body fine wear debris particles will form. These fine particles are released to the surrounding tissue. The body will then make efforts to neutralize and carry away these microparticles. This is done by foreign matter transporting giant cells. Transport is then effected into the remaining organism via the lymphatic vessel system.
The isolation and neutralization of such microparticles leads to significant alterations to the tissue. An osteolysis might occur, i.e. a loss of the periprosthetic bone portions of the prosthetic assembly. Nowadays, this alteration to the tissue in the adjoining bone by wear debris particles is considered an important cause of the loosening of endoprostheses. Grave alterations to the bone will then be recognized after a period of 10 to 20 years.
Attempts have been made already to reduce the generation of the microparticles with a view to increasing the stability in operation of the prosthetic assemblies.
A substantial improvement to joint functionality was achieved in decades of research work through an optimization of the pairs of sliding elements. Thus, for example, ceramic materials were introduced into endoprosthetics as mating elements that slide. In addition, progress was made particularly in the cementless implantation of prostetic hip joint assemblies. At this point, an important progress is the fact that a firm inlay in titanium or a titanium alloy is introduced in the region of the acetabular cup by locking it in the bony cup. A snugly fitting clamshell-shaped insert in polyethylene will then be placed in this artificial acetabular cup. Fixation of the artificial hip joint in the thigh region is effected by means of a stem which is inserted in the medullary space of the femur. A spherically shaped head is fixed on the end of the prosthetic stem by means of a cone fit.
In addition, the head may include a neck-type shape which has a conical seat to receive a cone of the prosthetic stem with different neck lengths being available. As a rule, the head is made of steel or a ceramic material.
When the artificial hip joint is operative a motion of the head is caused in the acetabular cup. Studies have shown that very fine debris particles may form whenever a step is made.
The acetabular cup or the insert will then undergo thinning and large volumes of wear debris will form in the course of years. Then, the reaction of the organism will frequently be such as to feed as many vessels and reactive tissues as possible to the wear debris region in order to cause foreign matter transporting giant cells to carry away the wear debris particles. As a rule, however, this does not work sufficiently. It is not a rare case that pasty amorphous substances which not only consist of wear debris, but also contain protein and fat constituents, are found in the new joint region after a long time since implantation, on one hand. On the other, thickening occurs in the surrounding vessels. This formation of new vessels and the attempt to carry away the foreign matter particles will then cause a loss of bone structures and a loosening effect.
Revision surgery will then restore a certain stability. A new loosening, however, will occur faster than can be established after the first implantation.
There are also other causes of loosening. Thus, for example, the bone cement serving as an anchoring material was also identified as being a cause of loosening. The result has been that implantations involving no cement are carried out more and more frequently. In doing so, attempts are made to achieve a primary stability which is as high as possible between the prosthetic components and the bone. To this effect, a shape fitting as snugly as possible is aimed at and a trial to obtain it is made by creating a seat which is as good as possible for the components to be anchored. Incorporation of the seat into the bone is mostly made by hand. On the other hand, it is also possible nowadays to design the prosthetic seat in the thigh in a very precise manner by using surgery robots.
Clinical experience has shown that the problems of wear debris formation are also encountered in cementless prosthetic assemblies and can cause the prosthetic components to loosen.
Accordingly, it is the object of the invention to provide a prosthetic hip joint assembly which has a decreased propension to loosen and an increased stability in operation.
The inventive prosthetic hip joint assembly comprises a femoral component with a stem for being anchored in the medullary space of a femur and a head on the distal end of the stem, an acetabular component for being anchored in the pelvic bone with an acetabular cup which pivotedly supports the head of the femoral component, and an articular capsule made of a flexible material, which is located at the femoral component at one end and at the acetabular component at the other end so as to allow the head to move in the acetabular cup and to prevent wear debris from the bearing zone of the head in the acetabular cup from migrating to the outside.
According to the invention, an artificial articular capsule bridges over the femoral component and the acetabular component and, thus, hides the bearing zone in the acetabular cup so that wear debris forming therein cannot exit from the prosthetic hip joint assembly. To this end, for example, the articular capsule may be sealingly connected to an insert of the acetabular cup or to the head and/or to a neck joining the head to the stem. The articular capsule then needs to be of a structure and/or material which permits sufficient movableness of the head which preferably is of a substantially spherical shape, in the acetabular cup. On the other hand, the material and the mounting of the articular capsule requires to be tight enough to prevent microparticles from migrating therethrough. Such microparticles may be a few ms or smaller in size.
The material used for the articular capsule, in particular, is a sheet or foil and/or tissue material. This may be a plastic and/or metallic and/or a natural material. Especially, the materials envisaged are PTFE fibres and/or PETP fibres. It should be particularly advantageous to use Goretex(copyright) (a PTFE material available from Gore) or Dacron(copyright) (a PETP material available from DuPont ((copyright): a registered trademark). Goretex(copyright) has already proved over the recent decades as a material for use in prosthetic vessel assemblies. It is a fibrous material or tissue which may be differing in pore sizes. In addition, it is possible to apply a coating at the inside and/or outside which will cause the pores to largely be sealed. Furthermore, prosthetic vessel assemblies in Dacron(copyright) (another fibrous or tissue material) have proved useful, too. Dacron(copyright), as a material for the articular capsule, may also be coated both at its inside and/or outside. In case of need, the articular capsule may consist of a material grown in vitro which has the characteristics of the natural articular capsule tissue.
To allow for a clearance of motion which is as large as possible the articular capsule may be formed as a corrugated bellows. For example, it may have a plurality of concertina-like corrugations. To prevent material ruptures in the region of the corrugations these may have reinforcements.
In addition, the articular capsule may have a bulged-out portion facing away from the bearing zone of the head in the acetabular cup. The bulged-out portion is below the bearing zone in an implanted prosthetic hip joint assembly. It forms a storage volume which is adapted to receive wear debris particles. Furthermore, a body-compatible substance may favourably exist here, which retains these wear debris particles and prevents them from receding back to the joint (fly-catching function).
It is further possible to configure the articular capsule in a way that it has a hose-like extension. Preferably, its end is provided with a closure which is adapted to be opened, e.g. a plug. It is through this plug that the prosthetic hip joint assembly may become accessible by means of a puncture and by advancing a catheter therethrough, and may be rinsed and cleaned, if required.
Especially in the case of complete encapsulation, some sort of lubrication may be effected by body-compatible substances inside the articular capsule so as to keep joint component wear debris as low as possible.
The articular capsule may also be securely fixed to the acetabular component, particularly to an insert of the acetabular cup, in a continuous groove. The articular capsule may be located in this groove by means of a bead and/or a locking collar. It is in the same way that the fixation of the articular capsule may be effected to the femoral component such as the head and/or neck. Also this one may have a continuous groove in which the articular capsule comes to rest with the aid of a bead or by means of a locking collar, in case of need. To prevent the artificial articular capsule from being pulled out of this mount a border projecting beyond the mount may include a bead which prevents the material from being pulled out from beneath the locking collar.
In a very particular, advantageous aspect of the invention, there is a pre-assembled unit comprising an insert, a head, and a capsule fixed to the insert at one end and to the head and/or neck at the other which has a seating cone in the neck. This pre-assembled unit constitutes the joint component proper which is placed in the hip joint area as the last component. After the implantation of the acetabular cup in the pelvis region and the stem in the medullary space of the femur, the encapsulated joint component is inserted in place. At this point, the insert with the articular capsule suspended thereon is introduced into the acetabular cup so as to subsequently position the head with the capsule suspended thereon on the cone of the stem. To make this operation easier, the neck may have a lateral slot through which the cone is inserted. The cone may then be pushed deeper into the seat and, thus, be secured therein.
A special configuration of the articular capsule""s outer surface may optimize the bonding behaviour of the woven. There is the assumption that mucous bursae will form and allow an improved movableness of the artificial hip joint with respect to the surrounding tissue. A particularly rough surface, e.g. a fibrous surface, may promote tissue bonding.
In addition, the prosthetic hip joint assembly may have integrated in it an electronic sensor to monitor the function of the prosthetic hip joint assembly. The data measured may be transmitted from inside the human body by means of a telemetering device which can be integrated or may be separately implanted in the prosthetic hip joint assembly. The functions of the joint which require to be monitored include, for example, the intactness of the articular capsule, the heat-up of the joint, and further include pathological motions of the femoral component and the acetabular component inside the bone, etc. This makes it possible to warn the patient of the possible failure or overstress of the prosthetic hip assembly.